A method for the study of membrane-associated biofilm populations of Escherichia coli ATCC 8739 using modified Swinnex filter units was developed. Biofilms were established under carbon limitation in a chemically defined simple salts medium. Cells, pressure filtered on to cellulose nitrate membranes in situ, were perfused from the sterile side. Steady-state conditions were attained at which the growth rate of the sessile cells could be demonstrated to be proportional to the flow rate of medium. The antibiotic susceptibility of these biofilms was examined by including ciprofloxacin within the perfusing medium. Susceptibility of the biofilms to ciprofloxacin was found to be affected not only by its concentration, but also by the growth rate of the bacteria.